provide a consistent type for 'customLimits'

lib/historyKeeper.js

@@ -68,7 +68,7 @@ module.exports.create = function (config, cb) {
         allowSubscriptions: config.allowSubscriptions,
         myDomain: config.myDomain,
         mySubdomain: config.mySubdomain,
-        customLimits: config.customLimits,
+        customLimits: config.customLimits || {},
         // FIXME this attribute isn't in the default conf
         // but it is referenced in Quota
         domain: config.domain